Method for producing a multilayer coextrudate and a film structure produced by this method

ABSTRACT

The invention relates to a method for producing a multilayer coextrudate, comprising a plastic layer that has release properties vis-à-vis adhesives, the release-property producing materials being disposed in the plastic layer. When the coextrudate is produced, a first web ( 2, 22 ) (support web I) is provided on the one side of which an adhesive layer ( 3, 23 ) is disposed, followed by the plastic layer ( 4,24 ) having the release properties, which in turn is linked with a second web ( 5, 25 ) (support web II). The release layer does not show substantial migration with respect to the adhesive and the release effect does not substantially change afterwards, especially during storage.

[0001] The invention relates to a process for the production of a multilayer coextrudate with a plastic layer that has release properties with respect to adhesives, where the materials producing the release properties are located within the plastic layer.

[0002] The Japanese patent application 59-122570 discloses a process in which a layer of adhesive and a release layer are coextruded, i.e. they are combined within an extruder die and leave this die as a web. A substrate web that is extruded from a separate die gap is then laminated onto the side with the layer of adhesive.

[0003] Working on the basis of this, the purpose of the invention is to indicate a process of the kind outlined above with which a composite material consisting of a web, an adhesive, a release layer and a further web for the release layer can be produced in an efficient way by the extrusion process.

[0004] In the solution to this problem proposed by the invention, a first web (substrate web I) is provided in production of the coextrudate on one side of which a layer of hotmelt adhesive is located, after which the plastic layer with the release properties follows, which is in turn bonded to a second web (substrate web II), where the release layer does not exhibit any material migration with respect to the adhesive and the release properties only change insignificantly afterwards, particularly during storage.

[0005] At least the basic structure of the film structure is produced in a single operation by this coextrusion process, with a substrate web being provided not only for the layer of hotmelt adhesive but also for the release layer.

[0006] It is very important in this context that the properties of the layer of adhesive and the release layer are coordinated in such a way that they do not influence each other substantially during and after the coextrusion process.

[0007] This makes sure that the separately set adhesive and release properties are not changed either in the production of the film structure or during its storage.

[0008] It has proved to be particularly favourable in this context if in accordance with a further development of the invention the layers of the coextrudate are coextruded simultaneously.

[0009] Particularly efficient production of the entire layer structure is achieved as a result.

[0010] In the process according to the Japanese application 59-122570, however, only the layer of adhesive and the release layer are coextruded, while the substrate web for the layer of adhesive has to be cooled first - which is done between two cooling rollers before it is laminated to the layer of adhesive. The release layer, on the other hand, is designed to be self-supporting and has nothing to do with a release layer in which materials producing the release properties are incorporated in a plastic layer.

[0011] In a further advantageous development of the invention, the other layers are extruded onto at least one of the substrate webs I or II.

[0012] It is, however, also possible in accordance with a further development of the invention that the layer of adhesive and the release layer are extruded between two webs.

[0013] In a particularly advantageous development of the invention, the layers are extruded simultaneously by the blown film process.

[0014] It is, however, also possible in accordance with an advantageous further development of the invention that the release layer and the layer of adhesive are coextruded by the blown film process, with the release layer forming the inner layer, and that these two layers are then collapsed and are bonded on each of their two outsides with one substrate layer for the layer of adhesive.

[0015] When the collapsed webs have been separated, the release layer is then provided with a substrate layer too.

[0016] In another advantageous development of the invention, the layer of adhesive, the release layer and a substrate web for one of the two layers are produced together by the blown film process, in which the substrate web is preferably provided on the inside and the layer structure is subsequently collapsed and bonded on both sides with a further substrate web.

[0017] In these configurations it must only be made sure that the layer of adhesive is not located so that it is free on the inside.

[0018] It is, however, also possible in accordance with a further development of the invention that the layers are produced by the cast film process.

[0019] In accordance with a further advantageous development of the invention, it is possible that a pre-produced web, to which the other layers are extruded in a single process operation, is provided as the substrate web.

[0020] This process is particularly favourable especially when a web is being used that is difficult or impossible to extrude at the same time, as all the other layers can be applied to the web that is acting as the substrate web in a single operation.

[0021] It is also very advantageous if in accordance with a farther development of the invention a plastic film, to which the other layers are extruded in a single process operation, is provided as the substrate web.

[0022] A further advantageous development of the invention is characterised by the fact that at least the substrate web is oriented.

[0023] The coextrudate receives favourable strength properties as a result.

[0024] It is also possible in accordance with an advantageous development of the invention that the entire film structure is oriented.

[0025] All the layers that can be oriented can help to increase the strength properties in this case.

[0026] An advantageous development of the process proposed by the present invention is characterised by the fact that the layer of adhesive and the release layer are coordinated with each other in such a way that the adhesive force is reduced by the release components to a minimal extent at most, as material migration of the release components into the adhesive is avoided.

[0027] In accordance with a further development of the invention, the reduction in adhesive force as measured by FINAT methods is supposed to be less than 20%.

[0028] This guarantees that the adhesive force does not decrease excessively and is no longer adequate for the planned application.

[0029] In accordance with an advantageous development of the invention, the release force between the release layer and the layer of adhesive is supposed to be between 5 and 50 cN/cm.

[0030] The main way this is achieved in accordance with a further advantageous development of the invention is that the release effect has been built up almost entirely when the extrusion process is carried out.

[0031] In a further advantageous development of the invention, the release components are produced using silicone graft and/or block copolymers.

[0032] The desired low migration of the release components into the neighbouring layer of adhesive is achieved by these release components.

[0033] In a further advantageous development of the invention, the release effect of the release layer is set in relation to the neighbouring adhesive during the extrusion process.

[0034] In accordance with an advantageous development of the invention, the adhesive force of the layer of adhesive can also be adapted to the subsequent application for the layer of adhesive with the relevant substrate web.

[0035] Depending on the planned application, the adhesive force is supposed to be set in a range from 1 to 20 N/25.4 mm, measured by AFERA methods, in accordance with a further development of the invention.

[0036] 1 to 4 N/25.4 mm are planned in this context for what are known as protective films, 5 to 10 N/25.4 mm for adhesive tapes and >10 N/25.4 mm for permanent labels.

[0037] Acrylate adhesives and/or rubber-based hotmelt adhesives are used as adhesives in accordance with an advantageous development of the invention.

[0038] In accordance with another advantageous development of the invention, thermoplastic elastomers—preferably styrene types such as SBS or SEBS—are used as adhesives, with which tackifiers (e.g. resins) are mixed during the extrusion process to control the adhesive properties.

[0039] It is also possible in accordance with a further development of the invention that elastomer blends such as EPDM/PP are used as adhesives, with which tackifiers (e.g. resins) are mixed during the extrusion process to control the adhesive properties.

[0040] In another advantageous development of the invention, metallocene catalysed polyolefins with a density of <0.880 g/cm³ are used as adhesives with the addition of tackifiers during the extrusion process.

[0041] In accordance with an advantageous development of the invention, a multilayer film structure has two substrate webs, between which a layer of adhesive and a release layer are located, where these two layers bond considerably more strongly to the substrate webs facing them than they do to each other.

[0042] The layer of adhesive and the release layer have solely their relevant function here without any significant support properties; these support properties are provided exclusively by the two substrate webs.

[0043] An advantageous further development of the film structure proposed by the present invention is characterised by the fact that at least two plastic films are provided, between which a layer of adhesive and a further layer that has release properties with respect to the adhesive are located.

[0044] In a further advantageous development of the invention, further layers are provided that are located on either one or both sides of the plastic films.

[0045] It has proved to be very advantageous if in accordance with a further development of the invention at least part of the coextrudate structure is oriented.

[0046] Considerable increases in strength are achieved by orienting the substrate web or the finished coextrudate.

[0047] It has also proved to be very advantageous if at least one of the substrate webs is made from paper.

[0048] It is, however, also possible in accordance with a further advantageous development of the invention if at least one of the substrate webs is made from metal.

[0049] At least one of the substrate webs can, however, be made from a pre-produced composite film as well, in accordance with a further advantageous development of the invention.

[0050] In a further advantageous development of the invention, at least one of the substrate webs is made from a nonwoven fabric.

[0051] In accordance with an advantageous development of the invention, the substrate web for the release layer is supposed to have a material thickness of 20 to 80 μ.

[0052] A material thickness of 60 to 200 μ is, on the other hand, contemplated for the substrate layer for the hotmelt layer.

[0053] Two embodiments of the invention are illustrated in the drawings:

[0054]FIG. 1 is a diagrammatic view of the structure of a film structure that consists of four layers and

[0055]FIG. 2 shows a further film structure consisting of five layers, again in a diagrammatic view.

[0056]1 in FIG. 1 is a film structure that consists of four layers and has been produced by the coextrusion process, i.e. all four layers were combined within an extrusion die.

[0057] A layer of adhesive 3, next to which a further layer 4 that has release properties with respect to the adhesive is located, is provided on one side of the first web 2. This release layer 4 is in turn bonded to a second web 5 (substrate web). This release layer 4 is a plastic to which agents with release properties have been added.

[0058] If both of the webs 2 and 5 are made from a coextrudable plastic, all four layers can be produced simultaneously, not only by the blown film process but also by the cast film process.

[0059] It is, however, also possible to coat the web 2 or the web 5 with the other layers subsequently by the coextrusion process.

[0060]FIG. 2 shows a further film structure 21 that consists of five layers. What is involved here is a web 22 that acts as the substrate layer for a release layer 24. This release layer 24 faces a layer of adhesive 23 that is bonded to a web 25 made of plastic. This web 25, which acts as the core layer, has a surface layer 26 on its side facing away from the layer of adhesive 23. Such a surface layer can also be provided on the web 22 that acts as the substrate layer for the release layer.

[0061] Different plastics can be used as the substrate layer for the release layer.

[0062] Good properties have, for example, been achieved with LDPE, LLDPE, HDPE, PP, mPE, PETP and PS, in each case 20 to 40 μ thick, while the actual release layer was 5 to 10 μ thick.

[0063] PP, OPP, PE, LDPE, LLDPE, mPE, PS and PET have been used very successfully for the web that supports the adhesive, with material thicknesses between 60 and 200 μ, depending on the material used. This web has in some cases been provided with an additional coating on the outside to improve printability.

[0064] The webs have in some cases been divided up into several individual layers made from different materials too.

[0065] Extrudable, permanently tacky adhesives based on hotmelts and polyolefins with appropriate tackifying additives have been used successfully as adhesives.

EXAMPLE 1

[0066] Use of SIS, SBS and SEBS block copolymers with melt indices of between 8 and 65 g/10 min at 200° C. and 5 kg. The styrene content of the polymers varies between 10 and 35%. The properties of the adhesive layer are controlled by the addition of resins and plasticisers, e.g. by means of aliphatic hydrocarbon resins, polyterpene resins, hydrolysed hydrocarbon resins, aromatic hydrocarbon resins, paraffin waxes, microcrystalline waxes, polyisobutylene and process oils.

[0067] Liquid components are processed into an extrudable form by carrying out a compounding operation first.

[0068] The extrusion temperatures vary between 100 and 240° C., avoiding excessively high shear forces at the same time.

EXAMPLE 2

[0069] Another way to produce the adhesive layer involves the inclusion of UV acrylates or UV-curing PSAs between the coextruded substrate and release layers by using melt transport technology.

[0070] The radiation-cured basic materials are, for example, acrylate copolymers with an integrated photoinitiator. UV cross-linking is carried out by the polymer substrate layer for the adhesive.

[0071] The acrylate copolymers can be modified by resins and plasticisers, while the tack of the adhesive layer can be regulated via the radiation dose.

[0072] Typical radiation wavelengths for cross-linking purposes are in the range between 250 and 260 nm (UVC).

[0073] Typical processing temperatures for the acrylate copolymers are between 110 and 150 ° C.

[0074] If a substrate web is used that is coated by the coextrusion process, it is possible to orient not only the substrate web alone but also the finished coextrudate in order to increase the strength properties.

[0075] If the entire structure is produced as a coextrudate, this coextrudate can also be oriented subsequently to increase the strength properties.

[0076] Orientation is only possible to a limited extent if substrate webs made from paper, nonwoven fabric or metal are being used.

[0077] If a pre-produced composite film is, on the other hand, used as the substrate web, orientation to increase strength is generally possible, depending on the material it contains. 

1. Process for the production of a multilayer coextrudate with a plastic layer that has release properties with respect to adhesives, where the materials producing the release properties are located within the plastic layer, wherein a first web (2, 22) (substrate web I) is provided in production of the coextrudate on one side of which a layer of hotmelt adhesive (3, 23) is located, after which the plastic layer (4, 24) with the release properties follows, which is in turn bonded to a second web (5, 25) (substrate web II), where the release layer does not exhibit any material migration with respect to the adhesive and the release properties only change insignificantly afterwards, particularly during storage.
 2. Process according to claim 1, wherein the layers of the coextrudate are coextruded simultaneously.
 3. Process according to claim 1, wherein the other layers are extruded onto at least one of the substrate webs I or II.
 4. Process according to claim 3, wherein the layer of adhesive and the release layer are extruded between two webs.
 5. Process according to claim 1, 2, 3 or 4, wherein the layers are extruded simultaneously by the blown film process.
 6. Process according to claim 1, 2, 3 or 4, wherein the release layer and the layer of adhesive are coextruded by the blown film process, with the release layer forming the inner layer, and wherein these two layers are then collapsed and are bonded on each of their two outsides with one substrate layer for the layer of adhesive.
 7. Process according to claim 6, wherein the layer of adhesive, the release layer and a substrate web for one of the two layers are produced together by the blown film process, in which the substrate web is preferably provided on the inside and the layer structure is subsequently collapsed and bonded on both sides with a further substrate web.
 8. Process according to claim 1, 2, 3 or 4, wherein the layers are produced by the cast film process.
 9. Process according to claim 1, wherein a pre-produced web, to which the other layers are extruded in a single process operation, is provided as the substrate web (2, 5; 22, 25).
 10. Process according to claim 9, wherein a plastic film, to which the other layers are extruded in a single process operation, is provided as the substrate web.
 11. Process according to one of the previous claims, wherein at least the substrate web is oriented.
 12. Process according to claim 11, wherein the entire film structure is oriented.
 13. Process according to one of the previous claims, wherein the layer of adhesive and the release layer are co-ordinated with each other in such a way that the adhesive force is reduced by the release components to a minimal extent at most, as material migration of the release components into the adhesive is avoided.
 14. Process according to claim 13, wherein the reduction in adhesive force as measured by FINAT methods is less than 20%.
 15. Process according to one of the previous claims, wherein the release force between the release layer and the layer of adhesive is between 5 and 50 cN/cm.
 16. Process according to one of claims 13, 14 or 15, wherein the release effect of the release layer has been built up almost entirely when the extrusion process is carried out.
 17. Process according to one of the previous claims, wherein the release components are produced using silicone graft and/or block copolymers.
 18. Process according to one of the previous claims, wherein the release effect of the release layer is set in relation to the neighbouring adhesive during the extrusion process.
 19. Process according to one of the previous claims, wherein the adhesive force of the layer of adhesive is adapted to the subsequent application for the layer of adhesive with the relevant substrate web.
 20. Process according to claim 19, wherein the adhesive force is set in a range from 1 to 20 N/25.4 mm, measured by AFERA methods, depending on the planned application.
 21. Process according to one of the previous claims, wherein acrylate adhesives and/or rubber-based hotmelt adhesives are used.
 22. Process according to one of the previous claims, wherein thermoplastic elastomers—preferably styrene types such as SBS or SEBS—are used as adhesives, with which tackifiers, e.g. resins, are mixed during the extrusion process to control the adhesive properties.
 23. Process according to one of the previous claims, wherein elastomer blends such as EPDM/PP are used as adhesives, with which tackifiers, e.g. resins, are mixed during the extrusion process to control the adhesive properties.
 24. Process according to one of the previous claims, wherein metallocene catalysed polyolefins are used as adhesives with the addition of tackifiers during the extrusion process.
 25. Process according to claim 24, wherein metallocene catalysed polyolefins with a density of <0.880 g/cm³ are used as adhesives with the addition of tackifiers during the extrusion process.
 26. Multilayer film structure produced in accordance with the process as described in one of the previous claims, wherein two substrate webs are provided, between which a layer of adhesive and a release layer are located, where these two layers bond considerably more strongly to the substrate webs facing them than they do to each other.
 27. Multilayer coextrudate produced according to the process as described in one of claims 1 to 24, wherein at least two plastic films (2, 5; 22, 25) are provided, between which a layer of adhesive (3, 23) and a further layer (4, 24) that has release properties with respect to the adhesive are located.
 28. Multilayer film structure according to claim 26 or 27, wherein further layers (26) are provided that are located on either one or both sides of the substrate webs and/or plastic films.
 29. Multilayer film structure according to one of claims 26 to 28, wherein at least part of the coextrudate structure is oriented.
 30. Multilayer film structure according to one of claims 26 to 29, wherein at least one of the substrate webs is made from paper.
 31. Multilayer film structure according to one of claims 26 to 29, wherein at least one of the substrate webs is made from metal.
 32. Multilayer film structure according to one of claims 26 to 29, wherein at least one of the substrate webs is made from a pre-produced composite film.
 33. Multilayer film structure according to one of claims 26 to 29, wherein at least one of the substrate webs is made from a nonwoven fabric.
 34. Multilayer film structure according to one of the previous claims, wherein the substrate web for the release layer has a material thickness of 20 to 80μ.
 35. Multilayer film structure according to one of the previous claims, wherein the substrate web for the hotmelt layer has a material thickness of 60 to 200μ. 